1. Field of the Invention
The present invention relates generally to modulated plasma generation and, more particularly, relates to a method and apparatus for modulated-bias plasma generation for etching of and for depositing one or more layers onto a substrate assembly.
2. State of the Art
A plasma is a collection of electrically charged and neutral particles. In a plasma, the density of negatively charged particles (electrons and negative ions) is equal to the density of positively charged particles (positive ions). A plasma also contains radicals. A radical is an atom or molecule with unsatisfied chemical bonding having an equal number of electrons and protons. All of the above-mentioned particles have a rate of decay. Consequently, by withdrawing a power source employed for plasma generation, the concentrations of these particles tend to decay.
Radicals are generally more abundant than ions in plasmas for two principal reasons. First, radicals are generated at a higher rate than ions, owing to a lower threshold energy and to ionization often being disassociative. Disassociation occurs if a collision between an electron and a polyatomic molecule results in a breakup of the molecule. Electron energy must be greater than molecular bonding energy for disassociation. Second, many radicals have a longer lifetime than many ions. For example, for a high-density plasma (a high-density plasma is typically defined as having an ion-electron density on the order of 10.sup.11 -10.sup.13 ions-electrons per cm.sup.3) operating at 1 mtorr, neutral to ion ratio is on the order of 100:1 to 1:1. Notably, some consider high density plasmas to have an ion-electron density greater than or equal to 10.sup.10 ions-electrons per cm.sup.3.
Plasma generation may be conducted by applying power to electrodes in a chamber of a reactor. In diode or parallel plate reactors, power is applied to one electrode to generate a plasma. In triode reactors, power is typically applied to two of three electrodes to generate a plasma.
In radio frequency (RF) plasma generation, for a diode reactor, a sinusoidal signal is sent to an electrode of a pair of electrodes. Conventionally, a wafer chuck or susceptor is the powered electrode. Examples of parallel plate reactors include the 5000MERIE from Applied Materials, Santa Clara, Calif.
A plasma source material, which typically includes one or more gases, is directed to an interelectrode gap between the pair of electrodes. Amplitude of the RF signal must be sufficiently high for a breakdown of plasma source material. In this manner, electrons have sufficient energy to ionize the plasma source material and to replenish the supply of electrons to sustain a plasma. The ionization potential, the minimum energy needed to remove an electron from an atom or molecule, varies with different atoms or molecules.
In a typical triode reactor, three parallel plates or electrodes are used. The middle or intermediate electrode is conventionally located in between a top and bottom electrode, and thus two interelectrode cavities or regions are defined (one between top and middle electrodes and one between middle and bottom electrodes). The middle electrode typically has holes in it. Conventionally, both the top and bottom electrodes are powered via RF sources, and the middle electrode is grounded. Examples of triode reactors are available from Lam Research, Fremont, Calif., and Tegal Corporation Ltd., San Diego, Calif.
Parallel plate and triode reactors generate capacitively coupled plasmas. These are conventionally "low density" plasmas (ion-electron density of less or equal to 10.sup.10 ions-electrons per cm.sup.3) as compared with high-density (also known as "hi density") plasmas which are generated by systems such as electron cyclotron resonance (ECR) and inductively coupled plasma (ICP). For ICP systems, an inductive coil (electrode) is conventionally driven at a high frequency using an RF supply. The inductive coil and RF supply provide a source power or top power for plasma generation. In ECR systems, a microwave power source (for example, a magnetron) is used to provide a top power. Both ICP and ECR systems have a separate power supply known as bias power or bottom power, which may be employed for directing and accelerating ions from the plasma to a substrate assembly or other target. In either case, voltage applied to a susceptor or wafer chuck (also known as the direct current (DC) bias) is affected by the bottom power (RF bias); whereas, current is affected by the top power.
In ICP systems, for example, ion density and ion energy distribution at surfaces in contact with a plasma depend on amplitude and frequency of a supplied RF bias. RF bias frequency can affect ion energy distribution (IED) at low frequencies. Such distribution may be bimodal, such as at lower frequencies characterized by a low energy tail of ions, and at higher frequencies characterized by a decrease in the low energy tail and by an ion energy distribution describable with a single energy level. Consequently, RF bias frequency can have a noticeable effect on a wafer surface or reactor walls.
In an abstract entitled "Effects of Bias Frequency on RIE Lag in Electron Cyclotron Resonance Plasma Etching System," H. H. Doh, K. W. Whang, Seoul National University, Korea, and C. K. Yeon, of LG Semicon Co. Ltd. Presented for the 43.sup.rd National Symposium (AVS), Philadelphia, Pa., Oct. 14-18, 1996, etch rate of SiO.sub.2 contact holes with sizes from 0.3 to 1.2 .mu.m using a C.sub.4 F.sub.8 +H.sub.2 ECR plasma etching system was examined. The parameters were: pressure (3 to 7.5 mTorr), microwave power (300 to 800 W), bias voltage (100 to 300 V), and bias frequency (100 kHz to 1 MHZ). As bias voltage and microwave power were increased, an improvement in RIE (Reactive Ion Etch) lag was reported. When the bias frequency was increased from 100 kHz to 800 kHz, maintaining the same bias voltage, an RIE lag improvement was reported even with a 30% H.sub.2 addition. IEDs were calculated using a Monte-Carlo particle-in-cell method, indicating bimodality below the frequency of 30 MHZ. As bias frequency increases from 100 kHz to 1 MHZ, the peak of low energy part decreases and the peak of high energy part increases. It was suggested that this change in IED is responsible for RIE lag improvement. Therefore, maintaining a high bias frequency may improve RIE lag.
While relatively high frequencies are employed for etching, these signals are pulsed at a lower frequency for providing a pulsed plasma. Conventionally, a pulsed plasma is pulsed by turning the driving power on and off. Typically, the driving power is turned on and off sufficiently rapidly to preclude extinction of the plasma during the off time. This turning on and off of power is inefficient with respect to power consumption and transients. It would be desirable to provide a pulsed plasma without the above-mentioned drawback.